Improved microstructure of alumina ceramics prepared from DBD plasma activated powders

Submicron Al₂O₃ powders were activated by Diffuse Coplanar Surface Barrier Discharge (DCSBD) plasma. The influence of the plasma treatment on the powder properties and their impact on the microstructure of dry and wet shaped ceramics were investigated. Raman and FTIR analyses of treated powders showed a substantial increase of the powder’s surface hydroxylation, surface cleaning, and the presence of adsorbed NO_x originating from the DCSBD. Sintering of the dry shaped plasma treated powders did not influence sintering behavior. On the other hand, the plasma treated powder was able to form stable water suspension without any chemical stabilization aid. Slip cast samples exhibited finer pore size distribution, a higher sinterability, and a finer final microstructure. The grain size of slip casted plasma treated powder was reduced by a factor of 1.7, which facilitated a grain size of 0.68 μm at the relative density of 99.54% t.d. obtained by pressure-less sintering.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Optical and mechanical properties of mn-doped transparent alumina and their comparison with selected rare earth and transient metal doped aluminas

Mn-doped translucent alumina prepared in this work was characterised in terms of densification process, microstructure, optical and mechanical properties. The manganese effect was compared with the influence of chromium, erbium and europium in previously prepared doped transparent alumina. Contrary to them, manganese accelerates both densification and grain growth. Real in line transmittance (RIT) at 632.8 nm and transmission/diffuse reflectance spectra in UV-VIS-NIR range of Mn-doped alumina were measured. The lower RIT values and rapidly decreased transmittance/reflectance below 800 nm are caused by relatively high absorption of light by Mn species and by increased grain size. The deep red emission of Mn⁴⁺ centres that almost fade at higher manganese concentration was indicated in photoluminescence spectra, with a decay time of ∼670 μs. The dopant introduction in all cases resulted in an increase in hardness of doped alumina up to 20 % compared to undoped alumina and a decrease in related fracture toughness.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

Microstructure and optical properties of alumina sintered from various phases

Self-synthesized and commercial alumina (boehmite, γ-Al₂O₃, α-Al₂O₃) powders were consolidated using an identical spark plasma sintering cycle, and optically translucent samples were obtained. The benefit of higher pressure is remarkable grain growth suppression. Additionally, the shorter dwell time at higher pressure advantageously leads to a reduction in grain size while conserving the optical transparency and without affecting the density.     

Luminescent Dy3+ and Dy3+/Cr3+ doped transparent Al2O3 ceramics: Microstructure and optical properties

Highly transparent Dy³⁺ and Dy³⁺/Cr³⁺ polycrystalline alumina ceramics were prepared with the real in-line transmittance (RIT) up to 55% (at λ = 632 nm), one of the highest values reported for luminescent rare-earth elements doped alumina. The RIT of doped alumina decreased more sharply with increasing mean grain size than predicted by the models for pure alumina. Co-doping with Dy³⁺ and Cr³⁺ resulted in a moderate increase of the RIT, which was independent on grain size. EDX analysis revealed that dysprosium segregated at grain boundaries and chromium was dispersed throughout the alumina matrix. A thermodynamic model for diffusion of multiple species in systems with multiple sorts of traps was proposed. The photoluminescence excitation and emission spectra of Dy³⁺-doped alumina showed peaks characteristic for Dy³⁺-doped materials. In co-doped Dy³⁺/Cr³⁺ alumina, Dy³⁺ emitted in the blue and yellow regions and Cr³⁺ in the red region, creating a suitable combination for obtaining white light.     

高纯度氧化铝粉体颗粒细化对烧结致密化及透光性能的影响

以法国Baikowski公司高纯度氧化铝粉体为参照,选取大连瑞尔精细陶瓷有限公司产超高纯度氧化铝粉体为研究对象。采用研磨处理,以改善国产氧化铝粉体的形貌、粒径及其分布。分别采用硅钼电炉中常压烧结和真空气氛下在1850℃烧结2种不同的烧结方式评价了研磨后粉体的烧结性能和用于制备半透明氧化铝陶瓷的可行性。结果表明:经过研磨改性处理后,粉体的粒径分布和比表面积接近于法国粉体;在1600℃常压烧结得到的氧化铝陶瓷达到理论密度的97%,具有均一的晶粒尺寸(～5μm)。添加MgO为烧结助剂,在真空下烧结得到了半透明氧化铝陶瓷,在波长为200～1100nm范围直线透过率最大值达到16%。     

Sintering Behavior of Nanocrystalline Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis

Powders of nanocrystalline zirconia doped with 3–30 mol% alumina have been synthesized using chemical vapor synthesis (CVS). Dense or mesoporous ceramics of small and narrowly distributed grain and pore sizes in the nanometer range are obtained via pressureless vacuum sintering. The microstructural development of the doped samples is strongly dependent on the alumina content. Sintering of zirconia samples with 3 and 5 mol% alumina at temperatures of 1000°C for 1 h results in fully dense, transparent ceramics with grain sizes of 40–45 nm and homogeneous microstructures.     

New insights in structural characterization of transparent ZnS ceramics hot-pressed from nanocrystalline powders synthesized by combustion method

Zinc sulfide transparent ceramics have been fabricated by hot pressing (HP) powders prepared by a newly developed combustion method. Chemical, structural and microstructural properties of powders and ceramics were characterized using different experimental techniques (XRD, SEM-EDS, laser granulometry, TEM, BET, FT-IR spectroscopy). ZnS powders were densified to full density by HP under vacuum atmosphere. The ceramics exhibit highly dense microstructure with mean grain size of 1 μm. TEM characterization identified, both in powders and ceramics, twins and simple stacking faults due to the aperiodic distribution of hexagonal domains. With optical transmission of the theoretical level (～75%), without absorption band (at 6 μm) and with negligible optical loss, in the 4–12 μm region, the ceramics exhibit better optical performances than standard grade CVD ZnS, and unprecedented performances for hot-pressed ZnS.     

Effects of calcination atmosphere on monodispersed spherical particles for highly optical transparent yttria ceramics

Highly sinterable powders are required for the fabrication of transparent ceramics. Here, we studied the effects of calcination atmosphere on the characteristics of monodispersed spherical Y₂O₃ powders, such as crystallite size and particle density, for high optical transparent ceramics. It was found that vacuum calcination around the crystallization temperature is the crucial step to eliminate intragranular pores in the spherical particle. The fast decomposition rate in a vacuum creates smaller crystallites, and the following higher calcination temperature results in the enhancement of pore elimination. The in‐line transmittance of the transparent Y₂O₃ ceramics, vacuum sintered at 1750°C, was improved by increasing the particle density of the as‐calcined powders. This result indicates that the high‐density starting particles effectively enhance the pore elimination during the fabrication of transparent Y₂O₃ ceramics.     

Electrical conductivity of sol–gel derived yttria-stabilized zirconia

A 3 mol% Y₂O₃ zirconia stabilized powder has been synthesized by destabilization of an aqueous zirconia sol prepared by the alkoxide hydrolysis method. The powder calcined at 500°C is ultrafine with tetragonal crystallites of about 8 nm, slightly agglomerated and with a narrow pore size distribution having an average pore size of 5.2 nm. Zirconia ceramics with density higher than 92%TD and grain size on the order of 100 nm have been obtained by uniaxial pressing at 500 MPa and vacuum sintering at 1000°C. Electrical conductivity of sintered samples, evaluated by complex impedance spectroscopy measurements, indicated that the zirconia stabilized with 3 mol% Y₂O₃ can potentially be used as an oxygen semipermeable dense membrane, but only at a relatively high temperature.     

Cubic Phase Stabilization of Translucent Yttria-Zirconia at Very Low Temperatures

Simultaneous decomposition of yttrium and zirconium alkozides was used to obtain an almost ideal mixture of powders of high surface activity. From this powder a translucent body, "Zyttrite," was made with a grain size of less than 1μ. The mixed oxide was consolidated into a stabilized body by firing at 1000°C for 30 min. This is indicative of a phenomenal increase in solid state reactivity. High-density fully stabilized cubic solid solutions of 6 mole % Y₂O₃-ZrO₂ were obtained on sintering at 1450°C. A typical specimen had a density of 5.9 g/cm³, a coefficient of thermal expansion of 11 × 10⁻⁶ in./in.°C, and a grain size of 2 to 4μ. Test pieces withstood 2200°C firings in air for 262 hr with grain growth of 35 to 50μ and virtually no change in composition, density, or stabilization.     

Transparent polycrystalline alumina obtained by SPS: Green bodies processing effect

Starting from a commercial slurry of high purity α-Al₂O₃, freeze-dried powders, cast, filter-pressed or cold isostatically pressed samples were produced. Resulting powders or green bodies showing different particles packing were densified by spark plasma sintering (SPS) to obtain transparent polycrystalline α-Al₂O₃. Microstructure and real in-line transmittance (RIT) after SPS were dependent on the particles packing quality. Avoiding large agglomerates, narrowing the pore size distribution, reducing the most-frequent pore size (D_mode) and avoiding macroscopic heterogeneities within the green bodies enabled high RIT values to be achieved in the visible and near-infrared spectrum. However, a limit was achieved in the preparation of green bodies for which reducing the D_mode had no more influence on the optical behaviour of samples sintered by SPS. Finally, pure α-Al₂O₃ samples presenting a high RIT_640 nm value of 53% were produced from all the green bodies obtained by the following techniques: filter-pressing, slip casting and cold isostatic pressing.     

Sintering of α-alumina for highly transparent ceramic applications

In this paper we present our results on the sintering of α alumina ceramics by hot isostatic pressing. It describes a simple method for obtaining precise relative density values on our almost 100% dense samples. Then, transparency results are discussed with respect to grain size and residual porosity measurements, comparing them to scattering calculations. Our results are not far from the best reported transmission values: almost 60% for a 1 mm thick sample. The other 40% diffuse light comes from the birefringence of alumina for the most part. However, they are transparent enough to see detailed structures at several kilometers through them.     

Formation of translucent nanostructured zirconia ceramics

In this work the mechanisms that affect the optical transparency of nanostructured translucent ZrO₂ ceramics are studied. The translucent ceramic samples were obtained from a low agglomeration nanosized powder at low pressure and low temperature sintering. Even low pressures cause structural changes and defect creation in the nanocrystals. Annealing was used to study the grain formation, structure and impact of defects. Significant changes in translucency were observed with increase in pore size. In order to further understand the defect creation, the obtained ceramics were doped with Er³⁺ ions and studied optically. Photoluminescence studies revealed a change in the ratio of green to red parts of the spectrum as well as luminescence quenching when samples were pressed into pellets. Additionally, grain and pore size dependence on annealing temperatures was studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy.     

Influence of the heating rate on grain size of alumina ceramics prepared via spark plasma sintering (SPS)

The dependence of grain size on the heating rate has been investigated for alumina ceramics prepared via spark plasma sintering (SPS). For this purpose, the local grain size has been determined via position-dependent microscopic image analysis, using two independent grain size measures (mean chord length and Jeffries grain size). For alumina ceramics prepared with heating rates between 5 and 100 °C/min (pressure 80 MPa, maximum temperature 1300 °C) it is found that for higher heating rates the grain size is smaller. However, the microstructural non-uniformity is so large that any grain size determination that does not take this non-uniformity into account becomes meaningless, because grain size gradients from the specimen periphery to the center are larger than the differences in grain size due to different heating rates. Temperature and pressure gradients are discussed as the most plausible reasons for the microstructural non-uniformity.     

Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis

Powder synthesis and ceramic processing methods have to be improved to take full advantages of new, improved properties of nanocrystalline ceramics. Sintered nanocrystalline ceramics of pure, undoped zirconia are formed from nanocrystalline powder of optimized quality obtained by the chemical vapor synthesis (CVS) method. The as-synthesized CVS ZrO₂ powder is nonagglomerated with a crystallite size of about 5 nm, narrow size distribution, and high crystallinity. On uniaxial compaction a transparent green body of ultrafine, uniform microstructure and narrow pore size distribution corresponding to the grain size distribution is formed, which is sintered under vacuum at 950°C into a transparent, fully dense ZrO₂ ceramic with a grain size of 60 nm.     

Transparent ZnS Ceramics by Sintering of High Purity Monodisperse Nanopowders

We report an efficient way of preparing transparent ZnS ceramics using the hot‐pressing technique. It has been found that the transparency is highly dependent on the purity and the grain size distribution of the starting ZnS powders. Highly pure and monodisperse ZnS powders have been obtained by posttreatment of the precipitated powders in a H₂S/N₂ flow for 2 h at 600°C. The obtained ZnS ceramics show fully dense and homogeneous microstructure with average grain size of ~1 μm and smooth grain boundaries, leading to an excellent transmission of around 70% in the mid‐ and far‐ IR regions. The preparation technique described in this study is highly reproducible.     

Stabilization of Nanoalumina Colloidal Slips

The processing of nanosized powders to produce dense components is a difficult task, mainly due to the strong agglomeration of nanoparticles. In this work, deagglomeration studies were carried out on sol–gel‐derived γ‐alumina suspensions through the addition of dispersant or dispersant in combination with ethylene glycol as binder and the proper wet ball‐milling. For this purpose, zeta‐potential, viscosity, and sedimentation studies were carried out, whereas dilatometric studies accompanied by XRD analyses were performed on the powders derived from the most stable suspensions. Sintering studies followed and the as received ceramics were evaluated by SEM. The optimum stabilization was achieved through the synergistic addition of 2.75 mg/g ammonium polymethacrylate with 1 wt% ethylene glycol. The respective powder, although it indicates a shift of the sintering start to higher temperature, leads to the densest ceramic with the finest (500 nm mean grain size) and the most homogeneous microstructure.